This invention relates generally to protective coatings applied to metals. More specifically, it is directed to a thermal barrier coating system for a metal substrate which is used in a high temperature environment.
Specially-formulated coatings are often used to protect metal parts which are exposed to high temperatures. Aircraft engines are made from such parts, as are land-based gas turbines. The combustion gas temperatures present in the turbine engine of an aircraft are maintained as high as possible for operating efficiency. Turbine blades and other elements of the engine are usually made of alloys which can resist the high temperature environment, e.g., superalloys, which have an operating temperature limit of about 1000.degree. C.-1150.degree. C. Operation above these temperatures may cause the various turbine elements to fail and damage the engine.
The protective coatings, often referred to as thermal barrier coatings or "TBC"s, effectively increase the operating temperature of the turbine engine by maintaining or reducing the surface temperature of the alloys used to form the various engine components. Most TBC's are ceramic-based, e.g., based on a material like zirconia (zirconium oxide), which is usually chemically stabilized with another material such as yttria. For a jet engine, the coatings are applied to various surfaces, such as turbine blades and vanes, combustor liners, and combustor nozzles.
Usually, the TBC ceramics are applied to an intervening bond layer ("bond coat") which has been applied directly to the surface of the metal part. The bond layer is often critical for improving the adhesion between the metal substrate and the TBC. Bond layers are typically formed from a material like "MCrAlY", where "M" represents a metal like iron, nickel, or cobalt. The bond layer may be applied by a variety of conventional techniques, such as PVD; plasma spray, or other thermal spray deposition methods such as HVOF (high velocity oxy-fuel). In the past, those skilled in the protective coating arts have generally concluded that the bond layers should be as dense and rough as possible. However, the deposition processes still undesirably resulted in coatings which were not dense, i.e., which could be characterized as "spongy".
The effectiveness of a TBC coating is often measured by the number of thermal cycles it can withstand before it delaminates from the substrate which it is protecting. In general, coating effectiveness decreases as the exposure temperature is increased. The failure of a TBC is often attributed to weaknesses or defects related in some way to the bond coat, e.g., the microstructure of the bond coat, or deficiencies at the bond coat-substrate interface or the bond coat-TBC interface.
It should be apparent from this discussion that new protective coating systems of increased quality are very desirable--especially for high performance applications, such as superalloy parts exposed to high temperatures and frequent thermal cycles. In addition to maintaining their integrity over a large number of thermal cycles, the coating systems should be compatible with conventional application equipment, e.g., various plasma spray techniques.